Various devices for internal fixation of bone segments in the human or animal body are known in the art. For instance, pedicle screw and/or hook systems are sometimes used as an adjunct to spinal fusion surgery, and which provides a means of gripping a spinal segment. Such systems may have a rod-receiving portion and an integral anchor portion, or may be provided with a separate anchor member, especially one that may be pivoted with respect to a rod-receiving member. Although pedicle screw systems, comprising a pedicle screw and a rod-receiving device, are primarily discussed herein, it is also possible to anchor a rod-receiving device to the spine with a different type of anchor member, such as a laminar hook. The pedicle screw of such a system includes an externally threaded stem and a head portion. The rod-receiving device (also referred to as a coupling device) couples to the head portion of the pedicle screw and receives a spinal rod. Two or more such devices are inserted into respective vertebrae and adjusted along the spinal rod to distract, de-rotate, and/or stabilize a spinal column, for instance to correct scoliosis or stabilize the spinal column in conjunction with an operation to correct a herniated disk. The pedicle screw does not, by itself, fix the spinal segment, but instead operates as an anchor point to receive the rod-receiving device, which in turn receives the rod. One goal of such a system is to substantially reduce and/or prevent relative motion between the spinal segments that are being fused.
Pedicle screw systems may be configured for minimally invasive surgery (MIS) techniques. MIS techniques require implantation, manipulation, and locking of the pedicle screw system through a very small incision in the patient's skin. It has been suggested that one possible advantage of a MIS approach is that it can decrease a patient's recovery time.
Conventional pedicle screw systems and even more recently designed pedicle screw systems have several drawbacks. Some pedicle screw systems include rather large and bulky assemblies to secure a rod, thus increasing opportunities for tissue damage in and around the surgical site during installation. Many of these systems also include set-screw type locking mechanisms or multi-part cap structures that require a significant portion of the assembly to be located above the rod, increasing the height or profile of the implants extending radially away from the spinal column, which may cause patient discomfort after implantation. Systems with set screws also lack a predetermined locking position, often resulting in overtorquing of the locking mechanism. Many systems also require a rod-receiving device to be coupled to the pedicle screw or other anchor device prior to implantation or include numerous components that must all be carefully assembled together, making these systems more difficult to install and maneuver in a spinal operation where MIS techniques are used.
Furthermore, assemblies with polyaxial fixation devices in the prior art ordinarily rely on downward force of the rod against the head of the fixation device to secure the fixation device against pivoting, so that the assembly, and specifically the rod receiving portion thereof, is provided with little support or stability prior to full locking of the rod.